The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 6,154,790; 6,232,870; 5,963,134; 5,838,253; 6,170,059 and 7,240,824;
U.S. patent publications applications 2003/0005316; 2006/0250235 and 2007/281664;
German patent application DE 10049965;
Japanese patent publications JP2006106831; JP2008109442; JP2007235653; JP2007180936; JP2007013244; JP2006148552 and JP 2006303747; and
Korean patent publication KR 100424626.